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Exchange Online Limits
Microsoft maintains an article that documents various limits for Exchange Online, including mailbox capacity limits, periodic message sending rate limits and total message size limits at https://learn.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/exchange-online-limits. This page is frequently updated as service definitions/features evolve over time.
Please note that currently all primary accounts receive an A5 license and all alumni accounts receive and A1 license. As of the writing of this article, that means that alumni only receive a 50GB quota compared to 100GB for primary accounts.